1. Field of the Invention
The present invention relates generally to transmission of data in a low signal to noise ratio environment, and more particularly to wireless transmission of data.
2. Description of Related Art
Currently, transmission of data in low signal to noise ratio environments or mediums, is done based on fixed frame length or fixed frame duration. A fixed frame duration is suitable for carrying voice data, as a voice encoders and decoders can be set to generate fixed length data samples (or a small set of sample lengths) within the frame interval. But for data traffic there is a large range of possible packet lengths and a mismatch between the packet length and a frame length must be compensated for in some way. Typically, packet segmentation and frame padding are used to force data to fit into a fixed frame size. Accordingly, it would be desirable to be able to transmit variable length data frames.
One environment in which variable length information packets is currently utilized the computer networking environment. As shown in prior art FIG. 1, a typical network packet structure includes a preamble 3 (for synchronization), a destination address 5, a packet size indicator 7, the data 9, and an error checking field 11. For wired computer network applications, it is typically assumed that the receiver can detect the beginning and end of each packet easily. This is because the signal to noise ratio on the wired channel of transmission is high.
A short preamble 3, provides the receiver time to find the beginning of the packet, achieve time synchronization, and read the packet size. The packet size indicator 7 indicates the size of the data field 9, and allows the system to in essence determine an end point of the packet in advance. The error checking field 11 is used to protect information in the packet, including the packet size indicator 7. If the error checking field 11 fails, the packet is thrown away.
In wireless networks, and other low signal to noise ratio environments, packet header in format on such as the size indicator 7 and error checking field 11 may be unreliable (may contain errors), and many packets of information would have to be discarded. Accordingly, such a package structure as shown in prior art FIG. 1 is not practical in wireless network and other low signal to noise ratio environments.
In particular, low signal to noise ratio environments necessitate the use of advanced error correcting codes. Coded symbols are typically received at a very low signal to noise ratio, making detection of the packet beginning and end difficult. Further, the packet size information in the packet header is coded, and cannot be read until after the packet boundaries are determined. Without reliable detection of the packet boundaries, data communication cannot proceed. For this reason, variable length packet transmisison in low signal to noise ratio situations has not been utilized.
In such wireless network systems, information is typically redundantly encoded so that if error bits are introduced, the information can be reconstructed without errors. In typical code division multiple access (CDMA) applications, if the original message includes xe2x80x9cNxe2x80x9d bits, the encoded message will include xe2x80x9c2Nxe2x80x9d or xe2x80x9c3Nxe2x80x9d bits.
Utilizing the apparatus and method of the present invention, variable length encoded data can be transmitted in low signal to noise ratio environments. By the apparatus and method of the present application, a particular data pattern is added to a beginning portion of encoded data to signify a beginning of the encoded data, and the same or another data pattern is added to end portion of the encoded data to signify the end of the encoded data. Further, since the encoded data itself may include the beginning or end data pattern or variations thereof and may therefore mistakenly indicate a beginning or end of the encoded data, the encoded data is first checked for such patterns. If such a pattern or similar patterns (likely to be mistaken for in low signal to noise ratio situations) are found within the encoded data, new patterns are substituted therefore. As such, variable length encoded data can be transmitted in a low signal to noise ratio environment, and can thereafter be easily decoded.